Object Survives Being Swallowed by a Star

Long before the Bible's tale of Jonah being swallowed by a whale, a small wannabe star emerged intact after being engulfed by a neighboring giant star, scientists say.

The victim was a brown dwarf, a failed star too small to sustain the nuclear reactions that ignites regular stars. The purpetrator was a red giant, an ancient star that once resembled our Sun but which puffed up to enormous size after its hydrogen fuel was depleted. The red giant has since expelled most of its gas into space and transformed into a dense, Earth-sized star called a white dwarfs.

Using the European Southern Observatory's Very Large Telescope, astronomers spied the binary system that remains: the brown and white dwarfs. The brown dwarf is thought to have survived being swallowed by its companion during the white dwarf's red giant phase.

The discovery, detailed in the Aug. 3 issue of the journal Nature, provides the first solid evidence that an object as small as a brown dwarf—which is just one step up from giant planet mass—can survive another star's red giant phase. Previously, only red dwarfs, stars with masses about a third that of our sun, have been known to withstand such events.

Two dwarfs

Called WD 0137-349, the system is located about 300 light-years from Earth. Its two dwarfs are separated by only a few thousandths the distance between Earth and the Sun and the objects rotate around in each other in about 2 hours.

In the past, the two objects were farther part, but the temporary engulfment by the red giant's gas envelope is thought to have slowed down the orbital speed of the brown dwarf, causing it to spiral inwards towards the center of its larger neighbor.

Although too small to become a star, the brown dwarf was still big enough to avoid vaporization when it was engulfed.

Had it been less than 20 Jupiter masses, "it would have evaporated during this phase," said lead author Pierre Maxted of Keele University in England.

Cosmic catalyst

But there's another reason the brown dwarf survived. Scientists think the failed star sped up its companion's red giant phase, the way enzymes speed up biological reactions while remaining unharmed. When it was engulfed, the brown dwarf amassed matter from the red giant's gas envelope, which it then radiated off into space. By doing so, it shortened its companion's red giant phase dramatically.

"Normal single red giants that don't swallow anything probably last about 100 million years, but in this system, it may have only lasted a few decades," study team member Matt Burleigh of the University of Leicester in England told SPACE.com.

The brown dwarf's reprieve from destruction is only temporary, however. Its orbit is slowly shrinking, and in about 1.4 billion years, it will be close enough for the white dwarf to siphon gas from surface. When this happens, the brown dwarf will slowly shrink in mass, while the accumulating matter on the white dwarf will trigger massive thermonuclear explosions called novas every few years.

History repeats

In about 5 or 6 billion years, what happened in WD 0137-349 will repeat in our solar system. Our sun will run out of hydrogen and become a red giant, expanding until its diameter is about the size of Earth's orbit. Unlike the brown dwarf, however, our planet is not expected to survive—at least not in its present form.

"It's an ongoing debate whether the Earth will be swallowed up or not," Burleigh said. "But what's for certain to happen is that the Earth's atmosphere and seas will be boiled off. Even if it doesn't quite get engulfed, Earth will be pretty much lifeless."

Several million years after the red giant phase, our Sun will shrink and become a white dwarf. At this point, the orbits of Jupiter and Saturn will double or even triple since the new white dwarf anchoring our solar system will be much less massive than our Sun is now.

Planets farther out might not be so lucky; they could become untethered and float off into interstellar space, Burleigh said.

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